Most color picture tubes presently being manufactured are of the line screen slit mask type. These tubes have spherically contoured rectangular faceplates with line screens of cathodoluminescent materials thereon and somewhat spherically contoured slit-apertured shadow masks adjacent to the screens. The mask slits are aligned in vertical columns with each column containing a plurality of slits which are vertically separated by bridge or web portions of the mask. The line screens in these tubes include peripheral borders having slightly curved sides and rounded corners.
Such line screen slit mask type tubes are screened by a photographic method that utilizes a line light source, such as disclosed in U.S. Pat. No. 4,049,451 issued to H. B. Law on Sept. 20, 1977. The use of a line light source to form continuous phosphor lines, however, has an inherent problem that must be solved. Because of the substantially spherical curvature of the shadow mask, the slit apertures of the mask, that are off the major and minor axes of the mask, are tilted with respect to the line light source image. If uncorrected, such tilting results in the formation of phosphor lines that are relatively ragged.
Several methods have been suggested to solve the problem caused by this tilting. One of these methods is disclosed in U.S. Pat. No. 3,888,673, issued to Suzuki et al. on June 10, 1975 and in U.S. Pat. No. 3,890,151, issued to Suzuki et al. on June 17, 1975. In the method of these patents, a shield plate is used in conjunction with a tilting or rocking line light source. As the shield plate is moved to expose various parts of the mask and screen, the light source is tilted so that it parallels the slits in the exposed part of the mask. Such method of screening not only requires several movable mechanical parts, but also is very time consuming since each exposed portion of the screen has to be exposed to the light source a sufficient time to sensitize a photosensitive screen layer.
In another method, the off-minor-axis mask aperture columns are bowed so that the apertures are less tilted with respect to a line light source. Patents illustrative of this concept are: U.S. Pat. No. 3,889,145, issued to Suzuki et al. on June 10, 1975; U.S. Pat. No. 3,925,700, issued to Saito on Dec. 9, 1975; and U.S. Pat. No. 3,947,718, issued to vanLent on Mar. 30, 1976.
In yet another method, a negative meniscus lens is located between a line light source and a shadow mask during screening to cause a rotation of the line light source image in a direction to decrease the aforementioned tilting of the slit image. Such method is disclosed in U.S. Pat. No. 4,078,239, issued to Prazak et al. on Mar. 7, 1979. As noted in this patent, the theoretical limit in reduction of tilting using the meniscus lens disclosed therein appears to be in the approximate range of 62% to 70% depending on tube sizes.
Recently, an improved line screen slit mask color picture tube has been suggested which has a more truly rectangular viewing screen than has previously been achieved in such tubes with spherically curved faceplates. It is particularly important in such improved tubes to form straight smooth phosphor lines on the sides of the screen. Therefore, it is not possible to use the aforementioned bowed apertured column concept to correct for aperture image tilting. Furthermore, although use of the aforementioned meniscus lens concept can provide some correction for light source image tilt, the theoretical limit to the amount of tilt correction still leaves something to be desired in achieving smooth phosphor lines at the sides of the screen.
Another solution to the tilting problem is presented in U.S. Pat. No. 4,516,841, issued to Ragland on May 14, 1985. This patent discloses the use of a generally cylindrical shaped lens between a line light source and a faceplate panel during exposure of photosensitive material on the panel. The longitudinal axis of the lens is oriented perpendicular to the longitudinal axis of the line light source. Because of the presence of the lens, the images of the line light source projected through the slits of the mask onto the photosensitive material at locations off the major and minor axes of the panel are rotated toward parallelism with the minor axis thereby resulting in exposure of straight smooth lines on the photosensitive material.
During screening with a line light source, it is common to move or oscillate the faceplate panel at a slow speed in a direction parallel to the line light source and the intended direction of the phosphor lines. This motion or oscillation compensates for the shadowing effect of the webs and provides a more uniform exposure of the lines. Unfortunately, when the cylindrical lens of the Ragland patent is used, this movement of the faceplate causes the projected image of the line light source to move sideways slightly where it lands in the corners of the faceplate. Because of this movement, the phosphor line areas that are exposed are somewhat wider than anticipated. It is therefore desirable to improve upon the method of the Ragland patent to solve this secondary tilting problem caused by movement of the faceplate panel.